1. Field of the Invention
The present invention relates to a laser diode driving circuit which is used as a light source, for instance, for an optical disc device, whereby the light output of the laser diode can be controlled stably for each of a plurality of output levels by means of a reference current selection system.
2. Description of the Prior Art
As a laser diode driving circuit for controlling the light output of a laser diode to a constant output level, there has been known type which makes use of an operational amplifier as shown in FIG. 1. The laser diode driving circuit is a circuit which is disclosed through publication in catalogs of laser diodes or the like.
In FIG. 1, LD is a laser diode, PD is a photodiode for monitoring the light output of the laser diode, and use is made of an arrangement in which the laser diode LD and the photodiode PD are built in a single package. In the package, one of the output surfaces of the laser diode LD is oriented facing the photodiode PD, and the light output P.sub.o is taken out to the outside from the other output surface.
The reference symbol A.sub.1 is an operational amplifier for controlling the forward current of the laser diode LD, and there is arranged an input signal setting circuit as will be described below.
Namely, between a line 1a for a positive power supply which provides a supply voltage V.sub.cc, and a line 1b for a negative power supply, there are connected in parallel a reference voltage setting circuit 2 which has two resistors R.sub.1 and R.sub.2 connected in series, and a photovoltaic current signal line which has a resistor R.sub.3 connected to a current line 3 for photovoltaic current I.sub.S.
The setting point 2a of the reference voltage V.sub.ref in the reference voltage setting circuit 2 is connected to the inverting input terminal 4a of the operational amplifier A.sub.1, and the output point 3a of the photovoltaic current signal in the photovoltaic current signal line is connected to the noninverting input terminal 4b.
The value of the reference voltage V.sub.ref that is set in the reference voltage setting circuit 2 is given by ##EQU1## The operational amplifier A.sub.1 controls the forward current I.sub.f of the laser diode LD so as to bring the voltage difference to zero between the reference voltage V.sub.ref that is input to the inverting input terminal 4a and the voltage I.sub.s.R.sub.3 of the photovoltaic current signal that is input to the noninverting input terminal 4b. Between the power supply line 1a and the output terminal 4c of the operational amplifier A.sub.1, there is connected a forward current circuit 5 of the laser diode LD. Further, R.sub.4 in the figure is a current restricting resistor.
When the current I.sub.f that flows in the laser diode LD reaches a predetermined "threshold current," the laser diode LD begins to oscillate to generate a light output P.sub.o. Upon receipt of the light output P.sub.o the photodiode PD generates a photovoltaic current I.sub.s which is proportional to the light output, and the voltage I.sub.s.R.sub.3 of the photovoltaic current signal is input to the noninverting input terminal 4b.
The operational amplifier A.sub.1 controls the forward current I.sub.f of the laser diode LD so as to bring to zero the difference between the two voltages that are input to both of the inverting and noninverting input terminals 4a and 4b by the feedback action of the light output P.sub.o from the laser diode LD to the photodiode PD.
When the difference between both input voltages to the input terminals 4a and 4b is controlled to be zero, the relation I.sub.s.R.sub.3 =V.sub.ref becomes valid, so that the output level of the light output P.sub.o of the laser diode is controlled in the sense that the value of the photovoltaic current I.sub.s is determined as ##EQU2##
Now, when the laser diode is used as the light source, for example, for an optical disc device, it is necessary to control its light output stably for a plurality of output levels corresponding to the write level, the read level, and others.
However, in the above driving circuit for the laser diode, resistor R.sub.3 is connected to the current line 3 of the photovoltaic current I.sub.s and the light output P.sub.o of the laser diode LD is controlled to a constant output level at the point where the value of the photovoltaic current I.sub.s has the value I.sub.s =V.sub.ref /R.sub.3 that is given by Eq. (2). Consequently, it is not possible to control the light output stably at each of the plurality of output levels, so that it has a problem in that it cannot be used as a light source for an optical disc device and the like.
Further, in the above driving circuit of the laser diode, the reference voltage V.sub.ref is set to be voltage-divided by the reference voltage setting circuit 2 in which two resistors R.sub.1 and R.sub.2 are connected in series, and the light output P.sub.o of the laser diode LD is controlled to a constant output level at the point where the value of the photovoltaic current I.sub.s becomes I.sub.s =V.sub.ref /R.sub.3 as shown in Eq. (2). Therefore, the light output cannot be controlled stably for each of the plurality of the output levels so that there was a problem in that the system could not be used as a light source for an optical disc device.